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From: Thomas Gleixner <tglx@linutronix.de>
To: Arul Jeniston <arul.jeniston@gmail.com>
Cc: viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, arul_mc@dell.com
Subject: Re: [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function.
Date: Mon, 19 Aug 2019 17:59:00 +0200 (CEST)
Message-ID: <alpine.DEB.2.21.1908191752580.2147@nanos.tec.linutronix.de> (raw)
In-Reply-To: <CACAVd4j60pn=td5hh485SJOcoYZ_jWQDQg2DVasSodPtsaupkw@mail.gmail.com>

On Mon, 19 Aug 2019, Arul Jeniston wrote:

> hi Tglx,
> > But for the above scenario:
> >
> > ktime_get()
> >         do {
> >                 seq = read_seqcount_begin(&tk_core.seq);
> >                 base = tk->tkr_mono.base;
> >                 nsecs = timekeeping_get_ns(&tk->tkr_mono);
> >
> >         } while (read_seqcount_retry(&tk_core.seq, seq));
> >
> > So if the interrupt which updates the timekeeper hits in the middle of
> > timekeeping_get_ns() then the result is discarded because the sequence
> > count changed and read_seqcount_retry() returns true. So it takes another
> > round which will be perfectly aligned with the updated time keeper.
> >
> 
> Do you mean to say the timekeeper  updates always happen from ktime_get?
> My point was, when one thread is in ktime_get other thread/isr updates
> timekeeper from different flow.

Timekeeper updates happen of course NOT from ktime_get(), but ktime_get()
is protected against concurrent updates via the seqcount. Simplified
without all the required barriers etc.

ktime_get()

    do {
       seq = tk->seq;
       if (seq & 1)
          continue;
       base = tk->base;
       nsec = get_nsec();
    while (seq != tk->seq);

update()

   tk->seq++;
   update_data();
   tk-<seq++;

It does not matter whether the update is an interrupt on the same CPU which
hits ktime_get() or whether it happens concurrent on a different CPU.

ktime_get() can never use inconsistent tk data for calculating the time.

Thanks,

	tglx

  reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-16  8:32 [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function. 'hrtimer_forward_now()' returns zero due to bigger backward time drift. This causes timerfd_read to return 0. As per man page, read on timerfd is not expected to return 0. This patch fixes this problem. Signed-off-by: Arul Jeniston <arul.jeniston@gmail.com> arul.jeniston
2019-08-16  9:05 ` [PATCH] FS: timerfd: [Trimmed unreadable long subject line ] Thomas Gleixner
2019-08-16 10:22 ` [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function Arul Jeniston
2019-08-16 10:45   ` Thomas Gleixner
2019-08-16 16:55     ` Arul Jeniston
2019-08-16 17:00       ` Arul Jeniston
2019-08-16 21:17         ` Thomas Gleixner
     [not found]           ` <CACAVd4hT6QYtgtDsBcgy7c_s9WVBAH+1m0r5geBe7BUWJWYhbA@mail.gmail.com>
2019-08-17 19:23             ` Thomas Gleixner
2019-08-19  6:07               ` Arul Jeniston
2019-08-19  8:04                 ` Thomas Gleixner
2019-08-19 14:25                   ` Arul Jeniston
2019-08-19 14:52                     ` Thomas Gleixner
2019-08-19 15:26                       ` Arul Jeniston
2019-08-19 15:59                         ` Thomas Gleixner [this message]
     [not found]                           ` <CACAVd4iRN7=eq_B1+Yb-xcspU-Sg1dmMo_=VtLXXVPkjN1hY5Q@mail.gmail.com>
2019-08-19 18:29                             ` Thomas Gleixner
2019-08-20  6:11                               ` Arul Jeniston
2019-08-20  8:49                                 ` Thomas Gleixner
2019-08-20  9:42                                   ` Arul Jeniston
2019-09-05  8:48                                     ` Arul Jeniston
2019-09-05 15:34                                       ` Thomas Gleixner
2019-09-06 16:36                                         ` Arul Jeniston
2019-09-07 14:38                                           ` Thomas Gleixner

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